Charles Miller has some thoughts on job satisfaction (or the lack thereof) - and I think (at least in the "knowledge worker" domain) you can divide people into roughly two groups, based on his last questions:

• Work is the sacrifice that we make so we can do the things (outside of work) that we enjoy, and that fulfill us
• We spend a lot of time at work. If we’re not doing something that we’re passionate about, that gives us some kind of fulfilment, we’re wasting a big part of our lives.

I'd bet good money that most of the "workaholics" you know fall into that second category. Me? I love what I do, which is why I spend so much time on it.